Vacuum metallizing of plastic and similar dielectric substrates is disclosed in various forms including U.S. Patents:
______________________________________ 2,992,125 Fustier 2,993,806 Fisher 3,118,781 Downing 3,914,472 Nakanishi 4,101,698 Dunning 4,131,530 Blum 4,211,822 Kaufman 4,215,170 Oliva ______________________________________
In addition, two reference books are:
Thin Film Phenomena, Kasturi L. Chopra, Robert E. Kreiger Publishing Company, Huntington, N.Y., 1979. pp. 163-189.
Handbook of Thin Film Technology, Leon I. Maissel and Reinhard Glang, McGraw-Hill Book Company, New York, N.Y., 1970., pp. 8-32 to 8-43.
U.S. Pat. Nos. 4,407,871, 4,431,711 and 4,713,143, assigned to assignee of the present invention and incorporated herein by reference, relate to metallizing of plastic articles and more particularly to the structure and spacing of discrete metal islands used to metallize, rather than a continuous metal film. The metallizing is performed utilizing the island coating system as detailed in the aforesaid patents. The system includes generally separate primer and basecoat coating layers, a metallizing layer and a topcoat layer. As disclosed in the above referenced patents, the coating layers contain non-volatile film forming polymers, generally in the range of 10-30%.
The coating layers of the island coating system are spray applied using compressed air to atomize the coatings. All of the coatings have been applied using a high volume, low pressure spray gun with organic solvents, generally at 70-90% by weight, as carriers for the coatings in order to be effectively deposited. If the mixture is not properly sprayed the aesthetic properties of the metallized appearance are not achieved. The material must be even, yet thick enough to cover surface irregularities and yet island formation must occur.
Following deposition, each coating layer is flashed at ambient temperature for twenty minutes to evaporate solvent. The coating layer is then cured for 30 minutes at an elevated temperature.
In addition to proper deposition of the coating layers, the appearance and performance of the commercial product, the conductivity of the metal layer, the corrosion resistance of the metal layer and/or the adhesion of the top coat all relate to the structure and spacing of the islands. The above referenced patents provide further teachings related to nucleation and film growth to the desired island structure and spacing that achieves these ends.
In U.S. Pat. No. 5,290,625, assigned to the assignee of the present invention and incorporated herein by reference, the above process is applied to aluminum parts. In a copending application, U.S. Ser. No. 08/248,649, filed the same day as the instant application, assigned to the assignee of the present invention and incorporated herein by reference, the technology for coating layer deposition is improved to allow film builds of 1.5 to 2.0 mils eliminating significant coating irregularities.
U.S. Pat. No. 4,431,711 shows the significant difference in performance to be obtained with a vacuum metallized flexible plastic product, top coated, where the metal particles are coalesced only to the island state instead of being allowed to coalesce as a thin continuous metallic film across which electrical conductivity is established.
The '143 patent adds to the process the step of etching the vacuum deposited material with a solvent which slowly dissolves or removes residual amounts of metal from the channels between the distinct islands. This clears the channels exposing additional bonding surfaces on the substrate for increasing the surface area of adhesion between the substrate and a protective dielectric top coat.
Typically, substrate surfaces include surface flaws and molding defects such as blow lines and knit lines. Such flaws can give a "satin" appearance to the product instead of a bright metallic appearance. Such flaws can be covered by the application of a primer layer followed by the basecoat layer as in the present island coating system to provide suitable thickness to correct for such substrate surface flaws. The primer layer provides a smoother surface for the basecoat layer. In general, a coating thickness of 2.0 mils or more for the primer and basecoat layer is usually sufficient to hide the substrate defects. Each individual layer is between 0.5 and 1.2 mils thick, with the combined thickness of at least 2.0 mils and often thicker.
However, having two separately applied layers adds to the cost and weight of the finished product as well as time of production. Two coating layers require two separate spray application steps and two separate flash and curing steps before metallizing. Hence, the cost and time of producing metallized objects is increased. A single thicker layer of either the primer or basecoat layer as formulated in the aforesaid patents did not adequately cover the surface. There was a significant increase of coating defects such as a "satin" appearance and poor appearance quality which can be described as a lack of brightness.
A thicker layer of primer only does not provide the proper surface chemistry upon which to form islands. A thicker layer of basecoat does not provide the necessary black background color such as that provided by the primer which is needed to give good appearance qualities.
An object of the present invention is to eliminate one of the two separate deposition steps of either the primer or basecoat. This would eliminate one set of flash and cure times increasing the efficiency of a production line making the metallized parts by at least 30%. A further object of the present invention is to reduce the cost and weight of a product while retaining the desired bright appearance, i.e. maintaining the aesthetic properties of the metallized appearance. Another object of the present invention is to reduce waste disposal of organic solvents.